Turbocharging is a means of greatly extending the power range and flexibility of internal combustion engines and has come to be an accepted practice, and in many cases a necessity, for heavy duty diesel engines of 200 or more horsepower. Turbocharging is also used to maintain power at increasing altitude, for instance in aircraft engines.
The effective application of a turbocharger to an internal combustion engine will normally increase power output 50-100 percent and reduce full load specific fuel consumption (sfc) by 5-10 percent. The reduced specific fuel consumption is attributed to two items. First, at a given speed the engine internal friction remains relatively constant even though the power output is increased considerably. This results in an effective improvement in mechanical efficiency. Second, if the efficiencies of the turbocharger components are high enough and the exhaust temperature of the engine on which the turbocharger is used is sufficiently high, there will result a positive pumping loop that adds to the net cycle output.
Turbocharging is normally thought of only as a means for increasing horsepower or decreasing full load specific fuel consumption. In the evolution of engines presently utilizing turbochargers, such as large heavy duty diesel engines, aircraft engines, racing engines and the like, this understanding has been adequate in that in the end application these engines are operated at or near full load for a large portion of the duty cycle. However, most applications do not require that the engine operate at or near full load for extended periods of time. In fact, in most applications, the engine generally is operated below 50% power and in many applications the engine operates well below 20% power during most of its operation. Examples of these applications are engines used in automobiles, light and medium trucks, generator sets, compressors, tractors, construction equipment and the like.
Engines operating at these low power settings are very inefficient. In a diesel engine, this inefficiency is a result of thermal efficiency decay as combustion temperature decreases and because the internal friction remains relatively constant regardless of load. In a gasoline engine, this inefficiency results from pumping loop loss increases with decreasing load and because the internal engine friction remains relatively constant regardless of load.
Thus, by utilizing a smaller "effective engine size," that is, an engine having a smaller displacement rate (the product of 1/2 displacement times engine speed for a four-cycle engine) by either a reduction in displacement, a reduction in operating speed or a combination of both, the part load fuel consumption may be improved. In gasoline engines, this improvement results from reduced engine friction and reduced pumping loop losses. In diesel engines, this improvement is a result of reduced engine friction and a higher thermal efficiency due to higher combustion temperatures.
In many applications, turbocharging may be used to permit the use of engines having smaller, "effective engine size." By turbocharging, it is relatively easy to obtain twice the naturally aspirated power per cubic inch of displacement and in some cases three times the power. However, attempts to turbocharge smaller engines have generally been unsuccessful. This failure can generally be attributed to the present design of turbochargers which are built around a journal bearing and a flat disc type thrust bearing. This type bearing system requires from one to three horsepower (depending upon the particular turbocharger and the speed required in the application) just to overcome friction. While this loss may be insignificant in applications where turbines are required to develop in excess of thirty horsepower for the compression process (typically engines of 200 or more horsepower), it becomes very significant when turbocharging engines having less than 100 horsepower. For example, where the turbocharger turbine power is 60-80 horsepower, a bearing friction loss of 2-3 horsepower is insignificant. However, in a smaller turbocharger where the turbine power is only 15 horsepower, a bearing friction loss of 2-3 horsepower, or more likely 4-5 horsepower due to the higher rpm at which the smaller turbochargers are operated, represents almost a third of the total turbine horsepower produced and is completely unacceptable.
The presently used bearing systems also require considerable radial and axial clearances to provide for oil flow and rotor stability. These clearances are translated into a relatively large clearance over the blading of the compressor and turbine rotors thereby affecting the efficiency of both the compressor and turbine. For example, the journal and disc thrust bearings, commonly used in present day turbochargers, may require a clearance of 0.015 inches between the turbine and compressor blades and the surround structure. Where the blade height is 1 inch, the clearance to blade height ratio is only 11/2 percent. However, where a smaller turbocharger is desired, having blade heights of, for example, 0.2 inches, a clearance of 0.015 inches between the blades and surround structure amounts to 71/2 percent of the blade height. Therefore, where a 0.015 inch clearance is acceptable in larger turbocharger applications, it is completely unacceptable when smaller turbochargers, are being designed. Therefore, in smaller turbochargers, this clearance becomes more and more critical to the overall performance of the turbocharger and ultimately to the performance of the engine.
The bearing systems now being used in turbochargers are lubricated with engine oil, although most bearing failures are the result of contaminated engine oil or lack of engine oil pressure during starts. Where high speed journal bearings are used in a conventional turbocharger, continuous oil flow is inherently required to provide shaft stability as well as to carry away heat generated by viscous friction. Oil flow is also required to carry away the heat transferred into the bearing system from the adjacent turbine (which operates at temperatures as high as 1600 degrees F.). Even if antifriction ball bearings were substituted for the journal bearings in conventional turbochargers, a continuous oil flow would be required to carry away heat transferred from the turbine. Thus, while lubrication is a necessity for the proper operation of present day turbochargers, lubrication also accounts for many of the failures. Further, continuous oil flow lubrication requires substantial plumbing and associated structure for providing the lubricant to the bearings.
The present day turbochargers have failed to efficiently control the flow of motive gases through the turbine. Presently, there are basically two methods used for controlling power output of the turbine. The first of these methods is by careful sizing of the turbine and turbine nozzle area so that at maximum engine operating speed and load the desired boost pressure will not be exceeded. The disadvantage of this method is that at low engine speeds the available boost pressure is limited and the response to demand is slow. The second method used for controlling the pressure through the turbine is the use of a "wastegate" in conjunction with a turbines nozzle sized to produce excessive turbine power at maximum engine speed and load. In this method, when the predetermined boost pressure is reached, the "wastegate" opens and bypasses a portion of the exhaust gases. While this method increases the available boost at the lower engine speeds and provides improvements in response, it is quite inefficient in that the bypassed, high pressure exhaust gas is simply wasted at the expense of increased engine back pressure. Additionally, at part load, when the turbocharger is essentially inoperative, the small nozzle area acts as a restriction to the exhaust and causes an increase in the pumping loop loss.
Therefore, a need has arisen for a turbocharger which can be efficiently operated to turbocharge both small and large internal combustion engines. The need is for a turbocharger having a bearing system which eliminates the problems heretofore experienced by continuous engine oil lubricated bearings and makes the most efficient use of the motive gases for driving the turbocharger turbine. Further, the bearing assemblies supporting the rotation of the compressor and turbine must facilitate the reduction of the required compressor and turbine rotor clearances.
The present design and method of production of prior art compressor housings for turbochargers have also presented a considerable problem.
Centrifugal flow compressors are one of the most widely used dynamic compressors in turbochargers. In this compressor type, air or an air-fuel mixture enters the compressor inlet, is channeled to the compressor rotor and is accelerated to near sonic speeds at a right angle to the inlet flow path. Increase in air pressure is accomplished by reducing the velocity of the accelerated gases as discharged from the tip of the compressor rotor blades. This process, known as diffusion, is more efficiently achieved by slowing down the gases without turbulence so that a large percentage of the velocity energy is converted into pressure energy, raising the static pressure.
To facilitate this diffusion process, turbochargers employing centrifugal compressors have normally included a compressor rotor wall closely following the contour of the compressor rotor blades from the blade leading edge to its outer tip. This compressor wall then extends past the outer tip of the blade to form one of two walls of the diffuser, then terminates to provide a circumferential gap through which the compressed gases are channeled into a circumferential chamber leading to the intake manifold of the engine. This wall, facing the compressor rotor blades and closely contoured to the rotor blades then extending outward, uniformly decreases the velocity of the gases after the gases leave the rotor blades and prior to their entry into the chamber leading to the engine. Thus, this wall structure greatly increases the static pressure generated by the compressor.
To form this structure, most turbocharger compressor housings have been sand-cast with the compressor wall cast in one piece with the compressor outer surround housing. This has normally been accomplished by using a sand core to form the circumferential chamber leading to the intake manifold of the engine. After casting, this sand core is dislodged to produce the chamber on the opposite side of the wall from the compressor in which gases are channeled off of the tips of the compressor rotor.
Although die-casting of the compressor housing would be substantially less expensive and more accurate than sand-casting, die-casting of an optimum design has not been possible because of the inability to use die-cast molds to form the circumferential chamber which channels the compressed gases to the intake manifold of the engine and at the same time form the diffuser wall. Because the variable area chamber is necessarily larger then the inlet gap through which gases are injected from the compressor rotor blades, die-casting an optimum design compressor housing has not been possible because of the inability to design molds that would form this passageway behind the wall facing the blades of the compressor rotor.
Where die-cast compressor housings are used, the wall normally formed in sand-cast compressor housings is merely deleted so that the molds may be brought together and parted to form the casting. However, without this wall, gases accelerated by the compressor rotor are prematurely dumped from the diffuser into the circumferential chamber leading into the engine intake manifold. As a result, this arrangement realizes a substantially lower compressor efficiency and thus lower performance.